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Page:Popular Science Monthly Volume 72.djvu/423

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419
INSECTS INJURIOUS TO AGRICULTURE

and that is that one can not count upon beneficial forms as a substitute for the methods of destruction commonly used in applied entomology. Their rôle does not consist of exterminating a species, but of maintaining a natural equilibrium, or of reestablishing it when it has been disturbed by human intervention. In such cases, their action can make itself felt in a more or less prompt way. It may happen that, immediately after having been imported into a country, they stop with an extraordinary rapidity the plague which they have been brought to combat. This was the case with Novius cardinalis, in California, and in different countries. It was also the case for Crypotolæmus montrouzieri, which made very rapid spread in Hawaii. It must be stated, nevertheless, that this is rather exceptional and that, in general, a number of years are required—the number varying according to the species and to the circumstances under which it is brought—before it can be completely naturalized in a given country, and before, thanks to its spread, it brings about a sufficient retardation of the multiplication of the plant-feeding species for which it is imported, to reduce it from the condition of a scourge to that of a supportable species.

The services that parasites and predaceous species render are sufficiently great so that there is no necessity for exaggerating them.

Far from lulling ourselves with illusions, we should keep on the watch and foresee the dangers with which other injurious species menace us, such as Icerya purchasi, which may any day invade Provence or Algeria on plants imported from Portugal and Italy.

There is no doubt that, however great may be the efficaciousness of a ladybird, like Novius cardinalis, it will be still better not to have the enemy at all than to be obliged to fight it by the intervention of its natural foe. We do not know that Novius cardinalis will with us develop with the success which marked its spread in California, in Portugal, and in Italy. We are ignorant whether the climatic influences or some parasite, recently adapted to this new strange host, will not limit its propagation and diminish its beneficial action. Finally, other plagues than Icerya menace us, and it is unfortunately certain that not all of these may be mastered by the equivalent of Novius cardinalis.

Confidence in the assistance which we can get occasionally from parasites and predaceous insects should not make us lose all prudence nor prevent us from seeking a guard against the perils which surround us, in organizing at our large ports an inspection and disinfection service like those which have been started at foreign ports, notably Hamburg, and in a general way taking every measure possible to protect our crops.